PRAM-1 is a phase-II, randomized, open-label, multicenter, multi-arm protocol. HIV-infected, clinically stable children, 24 months to 17 years of age, treated with the same antiretroviral therapy for 316 weeks, will be randomized to one of the three following treatment arms: 1) Zidovudine (ZDV) plus Lamivudine (3TC*); or 2) Stavudine (d4T) plus Ritonavir; or 3) ZDV plus 3TC* plus Ritonavir. Subjects will be stratified by CD4% <15% or 315%. The first eight subjects randomized to the d4T+Ritonavir and ZDV+3TC*+Ritonavir arms will participate in a real-time Phase-I pharmacokinetics study (16 subjects in total). The three arms will be evaluated primarily with respect to change in plasma HIV-1 RNA copy number from baseline to 48 weeks and with respect to safety and tolerance.